Production Gains from Re-Fracturing Treatments in Hassi Messaoud, Algeria

Abstract

Abstract Since 1990, over 200 hydraulic fracturing treatments have been performed in the Hassi Messaoud field in northeast central Algeria resulting in an average production increase of 5 m3/hr (750 bopd). The success of the stimulation program in this Cambrian age sandstone formation resulted from improved field practices of treating open-hole and slotted liner completions, and from the use of state-of-the-art fracturing equipment and engineering tools. After several years of production, the production rates of some fractured wells have declined due to several reasons: reservoir depletion, salt precipitation from formation water breakthrough or injection water breakthrough, and fracture conductivity damage due to barium sulfate scale and asphaltene deposition. Since 1996, re-fracturing treatments have been conducted on nine wells in order to restore production by placing another propped hydraulic fracture in the same target drain. The three case histories presented show exceptional, normal and marginal responses to the re-fracturing treatments. Explanations for the observed responses are presented and recommendations for the selection of new re-fracturing candidates in the Hassi Messaoud field are presented. Introduction The Hassi Messaoud oilfield, one of the giant oil fields of the world, is a thick sandstone reservoir in the northeastern part of central Algeria (see Figure 1). The field was discovered in 1956 and covers an area of approximately 2000 km2. Currently, the field has around 1000 wells. Figures 2 and 3 show a map of the field and a typical cross-section with the different productive horizons (drains) as found in the field. Hydraulic fracturing has been used as a technique to increase production of the oilfield since the early 1990s. Historic overviews of hydraulic fracturing activities have been presented in papers by McGowen et al.1 and by Bouazza et al.2. A significant amount of oil currently produced from the Hassi Messaoud field originates from wells that were hydraulically fractured. This is illustrated in Figure 4. At the end of 1999 proppant had been placed in approximately 200 wells in the field while injection tests had been conducted on approximately 250 wells. Figure 4 also illustrates that the production starts to decline as the number of fracturing treatments decreases. The main reason for the decrease in number of treated wells is the difficulty in selecting new candidates for hydraulic fracturing. The papers by McGowen et al.1 and by Bouazza et al.2 also give overviews of the production results of the treated wells. The fractured wells were divided into five categories, based on the results of decline-curve analyses2:Wells with normal declineWells with steep declineWells with slow clean-up.Wells with zero rate.Wells with unstable production behaviour. It was anticipated that re-fracturing some of the wells in categories 1 to 3 could potentially restore some of the declined production. Since 1996, re-fracturing treatments have been conducted on nine wells in the Hassi Messaoud field. There have been other wells that have been treated more than once, but the target drains were different in the different treatments. Strictly spoken these treatments can not be considered re-fracturing treatments if the target drains were not hydraulically fractured before.